In general, in order to achieve high level of absolute precision, the robot arms must be passed through various checks and adjustments in the manufacturing process before shipment. However, the precision of robot arm after long time of use is hard to maintain due to mechanical offset, or due to the deviation caused by maintenance of the robot arm (such as replacement of a motor or a gear set). Therefore, how to ensure the precision of the robot arm within the required range and directly calibrate the precision of the robot arm on the production line (in-line) is an urgent problem to be resolved for the industry.